Parallel tester capable of high speed plural parallel test

ABSTRACT

A test mode control circuit detects designation of a test mode in accordance with a combination of external control signals and address signals, and activates an internal period setting circuit. Internal period setting circuit generates a clock signal having a prescribed period when activated, and applies it to a control circuit. In accordance with the test mode designating signal from test mode setting circuit and the clock signal from internal period setting circuit, control circuit causes an internal address generating circuit to generate an internal address signal successively in synchronization with the clock signal, so that a word line of a memory array is selected.

This application is a Divisional of application Ser. No. 08/942,691filed Sep. 29, 1997, now U.S. Pat. No. 6,122,190 which is Divisional ofapplication Ser. No. 08/589,358 filed Jan. 22, 1996, now U.S. Pat. No.5,717,652.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor memory device and aparallel test therefor and, more specifically, to a semiconductor memorydevice and a structure of a parallel tester for performing test ofsemiconductor memory devices at a high speed.

2. Description of the Background Art

As memory capacity of a semiconductor memory device, specially of adynamic type RAM (hereinafter referred to as a DRAM) has been increased,time necessary for testing the semiconductor memory device has beenremarkably increased.

The reason why this problem occurs is that the more the storage capacityof the semiconductor memory device, the larger the number of word linesincluded therein, and therefore the longer becomes the time for writingand reading memory cell information while successively selecting theword lines.

The aforementioned problem is more serious in an acceleration test suchas a burn in test. In the burn in test, a semiconductor memory device isoperated under a high temperature and high voltage condition in order toreveal potential initial failure such as defect in gate insulating filmof an MOS transistor, which is a component of the device, defect in aninterlayer insulating film between interconnections, defect ininterconnection and defect caused by particles introduced during thesteps of manufacturing, and to eliminate defective products beforeshipment.

The above described burn in test is essential in maintaining quality ofthe products to be shipped. The increase in time for the test isdirectly related to increase of manufacturing cost of the semiconductormemory device.

The problem of longer test time is also experienced in a reliabilitytest such as a life test.

FIG. 45 schematically shows a structure of a conventional apparatus forperforming burn in test.

Referring to FIG. 45, on a test board TB, semiconductor memory devicesDR11 to DRmn are arranged in a matrix of m rows x n columns. Thesemiconductor memory devices DR11 to DRmn are connected to each other bya signal bus SG.

During testing, a control signal and a clock signal are output to testboard TB from a test signal generating circuit TA. The control signaland the clock signal are transmitted to each semiconductor memory deviceby the signal bus SG.

In the burn in test, first, data at a high level is written to eachmemory cell of the semiconductor memory devices DR11 to DRmn.Thereafter, a row address strobe signal /RAS and an address signal areapplied from test signal generating circuit TA to signal bus SG, and insemiconductor memory devices DR11 to DRmn, a word line is selected andsense amplifier circuit operates. The memory cell information amplifiedby the sense amplifier circuit is compared with the test data written inadvance, and thus malfunction of each semiconductor memory device isdetected.

The above described operation is continuously carried out for aprescribed time period under prescribed accelerating conditions.

FIG. 47 schematically shows a whole structure of a conventional dynamicsemiconductor memory device. Referring to FIG. 47, the dynamicsemiconductor memory device 1 includes a control circuit 18 receivingexternal control signals /WE, /OE, /RAS and /CAS applied throughexternal control signal input terminals 2 to 5 for generating internalcontrol signals; a memory cell array 7 in which memory cells arearranged in a matrix; an address buffer 9 receiving external addresssignals AO to Ai applied through an address signal input terminal 8 forgenerating an internal row address signal and an internal column addresssignal under the control of the control circuit 18; an internal addressgenerating circuit 10 for generating a refresh row address signal fordesignating a row to be refreshed during refreshing operation under thecontrol of control circuit 18; a multiplexer 11 for selectively passingany of the address signals from address buffer 9 and internal addressgenerating circuit 10 under the control of control circuit 18; and a rowdecoder 12 which is activated under the control of control circuit 18for decoding the internal row address signal applied from multiplexer 11to select a row of the memory cell array 7.

The signal /WE applied to external control signal input terminal 2 is awrite enable signal designating data writing. The signal /OE applied toexternal control signal input terminal 3 is an output enable signaldesignating data output. The signal /RAS applied to the external controlsignal input terminal 4 is a row address strobe signal for startinginternal operation in the semiconductor memory device and fordetermining active time period of the internal operation.

While the signal /RAS is active, circuits related to an operation ofselecting a row in the memory cell array 7 are activated. The signal/CAS applied to external control signal input terminal 5 is a columnaddress strobe signal, which activates a circuit for selecting a columnin memory cell array 7.

Semiconductor memory device 1 further includes a column decoder 13 whichis activated under the control of control circuit 18 for decoding aninternal column address signal from address buffer 9 and generating acolumn selecting signal for selecting a column of the memory cell array7; a sense amplifier sensing and amplifying data of a memory cellconnected to the selected row of the memory array 7; an IO gateresponsive to a column selection signal from column decoder 13 forconnecting the selected column of the memory cell array 7 to an internaldata bus a1; an input buffer 15 for generating an internal write datafrom external write data DQ0 to DQj applied to data input terminal 17 atthe time of data writing and transmitting thus generated data tointernal data bus a1, under the control of control circuit 18; and anoutput buffer 16 for generating external read data DQ0 to DQj frominternal read data read to internal data bus a1 at the time of datareading and outputting thus generated read data to data input/outputterminal 17, under the control of control circuit 6.

Referring to FIG. 47, the sense amplifier and the IO gate arerepresented by one block 14. Input buffer 15 is activated when thesignals /WE and /CAS are both at the active state of low level andgenerates the internal write data. Output buffer 16 is activated inresponse to the activation of the output enable signal /OE.

As described above, the operation of the DRAM is controlled by theaforementioned external signals /WE, /OE, /RAS and /CAS as well asaddress signals A0 to Ai.

Therefore, even in the burn in test, these signals are applied from testsignal generating circuit TA to each of the semiconductor memory devicesDR11 to DRmn.

In the above described burn in test, in order to suppress increase intest time even when the memory capacity of each semiconductor memorydevice is increased, the control signal /RAS transmitted from testsignal generating circuit TA to signal bus SG shown in FIG. 45 may bechanged at high speed, so as-to shorten the time necessary for the wordline to be selected.

However, a large number of semiconductor memory devices DR11 to DRmn areconnected to signal bus SG, and there is a large parasitic capacitanceCp at signal bus SG, as shown in FIG. 45. Therefore, because ofinterconnection resistance and the large parasitic capacitance of thesignal bus SG, there is a signal propagation delay, and hence increasein speed of changing said signal is limited.

FIG. 46 shows the change in the control signal /RAS and of the addresssignal on signal bus SG, as an example.

FIG. 46(A) shows an ideal signal waveform on signal bus SG, and FIG.46(B) shows a signal waveform on signal bus SG in the conventional burnin test. As shown in FIG. 46(A), in the ideal state, the signal /RASchanges with a prescribed rise time and a prescribed fall time, notinfluenced by the signal propagation delay. The address signal requiresa set up time Ts and a hold time Th with respect to the signal /RAS. Theset up time Ts is necessary for the address signal to be in theestablished state before the fall of the signal /RAS. The hold time This necessary for maintaining the established state of the address signalfrom the fall of the signal /RAS.

If the signal bus SG has large parasitic capacitance Cp, the rise timeand the fall time of the control signal /RAS become longer because ofthe signal propagation delay on signal bus SG, and the waveform isdeformed as shown in FIG. 46(B). Therefore, it becomes impossible tochange the control signal /RAS at high speed.

At this time, the speed of change of the address signal also becomesslower. In order to ensure the address set up time Ts, it is necessaryto change the address signal at an earlier timing than the timing ofchange of the address signal in the ideal waveform (FIG. 46(A)). Sincethe address signal is changed while the control signal /RAS is at aninactive state of high level, the period in which the control signal/RAS is inactive becomes longer than in the ideal waveform.

As a result, the time period for one cycle (word line selection cycle)of the burn in test 1 becomes longer, the word lines cannot besuccessively selected at high speed, and hence the time necessary forthe burn in test cannot be reduced.

In the burn in test, prescribed memory information is written in eachmemory cell in advance, and it is compared with an expected value whichis the information successively read and written by successivelyselecting the word lines, so that data bit error is detected anddefective products can be found. If it is difficult to change thecontrol signal /RAS at a high speed as described above, the timenecessary for the test is also increased since the time for the cycle ofwriting a signal, which is the aforementioned expected value in advanceis increased.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide asemiconductor memory device capable of executing test mode operationsuch as a burn in test at a high speed.

Another object of the present invention is to provide a parallel testercapable of reducing time when a plurality of semiconductor memorydevices are tested, as the aforementioned signal with respect to each ofthe semiconductor memory devices can be changed at high speed.

A still further object of the present invention is to provide asemiconductor memory device capable of significantly reducing time forwriting initial memory information, which will be an expected value inan operation test to each memory cell, and to provide a method ofoperation therefor.

Briefly speaking, in the present invention, clock generating circuitrywhich is activated in a test mode for generating an internal clocksignal is provided in a semiconductor memory device, and the clocksignal is utilized as a word line selecting operation activating signal.

More specifically, the semiconductor memory device in accordance withone aspect of the present invention includes a memory cell arrayincluding a plurality of memory cells arranged in a matrix; a clockgenerating circuit responsive to one external test mode designatingsignal for generating a clock signal of a prescribed period while thetest mode designating signal is active; an internal address generatingcircuit responsive to the test mode designating signal and the clocksignal for successively generating internal address signals insynchronization with the clock signal; an address signal switchingcircuit receiving the external address signal and the internal addresssignal, for outputting either of these in response to the test modedesignating signal; and a row selecting circuit operating insynchronization with the clock signal for selecting, in response to anoutput from the address signal switching circuit, the corresponding rowof the memory cell array.

According to another aspect of the present invention, a clock generatingcircuit which is activated in the test mode for generating an internalclock signal in synchronization with an externally applied clock signalis provided in the semiconductor memory device, and the internal clocksignal is utilized as the word line selecting operation activatingsignal.

More specifically, the semiconductor memory device includes a memorycell array including a plurality of memory cells arranged in a matrix; afirst clock generating circuit responsive to an external test modedesignating signal, for receiving an external clock signal andgenerating a first internal clock signal in synchronization with theexternal clock signal while the test mode designating signal is active;an internal address generating circuit responsive to the test modedesignating signal and the first internal clock signal, for successivelygenerating internal address signals in synchronization with the firstinternal clock signal; an address signal switching circuit receiving theexternal address signal and the internal address signal for outputtingeither of these in accordance with the test mode designating signal; anda row selecting circuit operating in synchronization with the clocksignal and selecting a corresponding row of the memory cell array inaccordance with the output from the address signal switching circuit.

According to a still further aspect of the present invention, aninternal clock signal generating circuit which is activated in the testmode, receiving an external clock signal and generating an internalclock signal which is obtained by multiplying the external clock signalis provided in the semiconductor memory device, and the internal clocksignal is utilized as the word line selecting operation activatingsignal.

More specifically, the semiconductor memory device includes a memorycell array including a plurality of memory cells arranged in a matrix; afirst clock generating circuit responsive to an external test modedesignating signal for receiving an external clock signal and generatinga first internal clock signal by multiplying said external clock signalwhile said test mode designating signal is active; an internal addressgenerating circuit responsive to an operation mode designating signaland the first internal clock signal for successively generating internaladdress signals in synchronization with the first internal clock signal;an address signal switching circuit receiving the external go addresssignal and the internal address signal for outputting either of these inresponse to the operation mode designating signal; and a row selectingcircuit operating in synchronization with the clock signal for selectinga corresponding row of the memory cell array in accordance with theoutput from the address signal switching circuit.

According to a still further aspect of the present invention, when aplurality of semiconductor memory devices divided into a plurality ofsubgroups are to be subjected to operation test parallel to each otherand in synchronization in accordance with an external clock signal, acircuit receiving the external clock signal for generating synchronizedinternal test clock signals for each subgroup is provided, and thesemiconductor memory devices are tested in parallel at high speed.

More specifically, a parallel tester for performing operation test of aplurality of semiconductor memory devices in parallel and insynchronization in accordance with an externally input external clocksignal includes an internal test clock generator provided for eachsubgroup of a plurality of semiconductor memory devices divided into aplurality of subgroups, receiving the external clock signal forgenerating a synchronized internal test clock signal; and a data busline for transmitting the internal test clock signal to each of thesemiconductor memory devices of the subgroup.

According to a still further aspect of the present invention, when datais to be written to the memory cell of the semiconductor memory device,the well potential of the memory cell transistor constituting the memorycell is controlled independent from the cell plate potential of a memorycell capacitor, so that memory information is written collectively.

More specifically, the semiconductor memory device according to thisaspect includes: a memory cell array including a plurality of wordlines, a plurality of bit line pairs crossing the plurality of wordlines and a plurality of memory cells connected to the word lines andthe bit line pairs, each memory cell including a first electrode, asecond electrode opposing to the first electrode with an insulating filminterposed, and a memory cell transistor of a first conductivity typeformed in a well of a second conductivity type, having its gateconnected to the word line and connecting/disconnecting the secondelectrode to the bit line; the semiconductor memory device furtherincludes a first interconnection commonly connected to the wells of thesecond conductivity type of the memory cells, a second interconnectioncommonly connected to the first electrodes of the memory cells and apotential controlling circuit capable of controlling the potentials ofthe first and second interconnections independent from each other.

Therefore, an advantage of the present invention is that accelerationtest is possible without any influence of distortion in waveform of anexternal clock signal, since the clock generating circuit generates aninternal clock signal of a prescribed period in accordance with oneexternal test mode designating signal and the semiconductor memorydevice operates in accordance with the internal clock signal.

Another advantage is that acceleration test is possible without anyinfluence of the distortion in waveform of the external clock signal,since, in accordance with an external test mode designating signal, afirst internal clock signal in synchronization with the external clocksignal is generated and semiconductor memory device operatesaccordingly.

A further advantage is that acceleration test is possible without anyinfluence of distortion in the waveform of the external clock signal,since, in accordance with an external test mode designating signal, aninternal clock signal is generated by multiplying an external clocksignal and the semiconductor memory device operates accordingly.

A further advantage of the present invention is that even when a numberof semiconductor memory devices are to be tested in parallel, distortionin clock signal applied to each of the semiconductor memory devices canbe suppressed, since the plurality of semiconductor memory devices to betested in parallel are divided into a plurality of subgroups, and aninternal test clock generating circuit for generating, in response to anexternal clock signal, a synchronized internal test clock is providedfor each subgroup of the semiconductor memory devices.

A still further advantage of the present invention is that memoryinformation for testing can be written collectively to a plurality ofmemory cells, since well potentials and the plate potential of eachmemory cell is controlled independent from each other by a potentialcontrol circuit.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram showing a structure of asemiconductor memory device in accordance with a first embodiment of thepresent invention.

FIG. 2 is a schematic diagram showing a first example of an internalperiod setting circuit in accordance with the first embodiment of thepresent invention.

FIG. 3 is a schematic diagram showing a second example of the internalperiod setting circuit in accordance with the first embodiment of thepresent invention.

FIG. 4 is a schematic diagram showing a control gate circuit inaccordance with the first embodiment of the present invention.

FIG. 5 is a diagram of waveforms showing an operation of the firstembodiment of the present invention.

FIG. 6 is a schematic block diagram showing a structure of asemiconductor memory device in accordance with a second embodiment ofthe present invention.

FIG. 7 is a schematic block diagram showing a structure of an internalperiod setting circuit and of a period storing circuit in accordancewith the second embodiment of the present invention.

FIG. 8 is a schematic diagram showing a structure of the period storingcircuit in accordance with the second embodiment of the presentinvention.

FIG. 9 is a schematic block diagram showing a structure of an internalvoltage lowering circuit.

FIG. 10 shows correspondence between an external power supply voltageand an internal power supply voltage during acceleration test.

FIG. 11 is a schematic block diagram showing a structure of asemiconductor memory device in accordance with a third embodiment of thepresent invention.

FIG. 12 is a block diagram showing details of a control circuit and atest mode control circuit in accordance with the third embodiment of thepresent invention.

FIG. 13 is a schematic block diagram showing a structure of asemiconductor memory device in accordance with a fourth embodiment ofthe present invention.

FIG. 14 is a schematic block diagram showing a structure of an internalsynchronizing circuit in accordance with the fourth embodiment of thepresent invention.

FIG. 15 is a partially omitted circuit diagram showing a structure of aclock buffer 91 shown in FIG. 14.

FIG. 16 is a partially omitted circuit diagram showing a structure of aclock buffer 96 shown in FIG. 14.

FIG. 17 is a schematic diagram showing a structure of a phase comparatorshown in FIG. 14.

FIG. 18 is a timing chart showing an operation of the phase comparator92 shown in FIG. 14.

FIG. 19 is another time chart showing an operation of the phasecomparator 92 shown in FIG. 14.

FIG. 20 is a schematic diagram showing structures of a charge pump 93and a loop filter 94 shown in FIG. 14.

FIG. 21 is a partially omitted circuit diagram showing a structure of avoltage controlled delay circuit shown in FIG. 14.

FIG. 22 is a timing chart showing an operation of a DLL circuit shown inFIG. 14.

FIG. 23 is a timing chart showing an operation of the fourth embodimentof the present invention.

FIG. 24 is a schematic block diagram showing a structure of an internalsynchronizing circuit 70 in accordance with a fifth embodiment of thepresent invention.

FIG. 25 shows structure and operation of the fifth embodiment of thepresent invention in which (a) is a schematic block diagram showing thestructure of the fifth embodiment, and (b) is a timing chart showing theoperation of the fifth embodiment.

FIG. 26 is a schematic block diagram showing a structure of asemiconductor memory device in accordance with a sixth embodiment of thepresent invention.

FIG. 27 is a schematic block diagram of a main portion of thesemiconductor memory device in accordance with the sixth embodiment ofthe present invention.

FIG. 28 is a timing chart showing an operation of the sixth embodimentof the present invention.

FIG. 29 is a schematic block diagram showing a structure of asemiconductor memory device in accordance with a seventh embodiment ofthe present invention.

FIG. 30 is a schematic block diagram showing a structure of asemiconductor memory-device in accordance with an eighth embodiment ofthe present invention.

FIG. 31 is a schematic block diagram of a main portion of thesemiconductor memory device in accordance with the eighth embodiment ofthe present invention.

FIG. 32 is a schematic block diagram showing a structure of an internalmultiplication circuit in the semiconductor memory device in accordancewith the eighth embodiment of the present invention.

FIG. 33 is a timing chart showing an operation of the eighth embodimentof the present invention.

FIG. 34 is a schematic block diagram showing a structure of a paralleltester in accordance with a ninth embodiment of the present invention.

FIG. 35 is a cross section showing a structure of a memory cell in thesemiconductor memory device in accordance with a tenth embodiment of thepresent invention.

FIG. 36 is a schematic diagram showing an equivalent circuit of thememory cell shown in FIG. 35.

FIG. 37 is a cross section showing the flow of operation in accordancewith the tenth embodiment of the present invention.

FIG. 38 is another cross section showing the flow of operation of thetenth embodiment of the present invention.

FIG. 39 is a schematic block diagram of a main portion of thesemiconductor memory device in accordance with the tenth embodiment ofthe present invention.

FIG. 40 is a schematic block diagram showing a structure of the tenthembodiment of the present invention.

FIG. 41 is a schematic block diagram showing a main portion of thesemiconductor memory device in accordance with an eleventh embodiment ofthe present invention.

FIG. 42 is a schematic block diagram showing a structure of asemiconductor memory device in accordance with a twelfth embodiment ofthe present invention.

FIG. 43 shows memory pattern of memory cells after implementation of thetwelfth embodiment of the present invention.

FIG. 44 is a schematic block diagram showing a main portion of asemiconductor memory device in accordance with a thirteenth embodimentof the present invention.

FIG. 45 is a schematic block diagram showing a structure of aconventional parallel tester.

FIG. 46 is a diagram of waveforms showing a clock signal in theconventional parallel tester.

FIG. 47 is a schematic block diagram showing a structure of aconventional semiconductor memory device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 11 schematically shows a whole structure of a semiconductor memorydevice in accordance with a first embodiment of the present invention.

Referring to FIG. 1, the semiconductor memory device 1 includes acontrol circuit 18 receiving external control signals EXT./WE, EXT./OE,EXT./RAS and EXT./CAS for generating various internal control signals;an internal period setting circuit 20 receiving an external test modedesignating signal EXT.BI for starting output of an internal clocksignal CLK, and in accordance with an external internal clock periodcontrol signal FS for changing the period of the internal clock signalCLK to be output; a control gate circuit 22 receiving the internal clocksignal CLK for outputting, in accordance with the external controlsignal EXT.BI, the internal clock signal CLK to an external terminal 5to which the signal /CAS is input and to a control circuit 6; and abuffer input signal control circuit 24 receiving outputs from a senseamplifier circuit and from the input/output control circuit 14 and theinternal clock signal CLK, for outputting the clock signal CLK to theoutput buffer 16 while the external control signal EXT.BI is active, andfor outputting the output signals from the sense amplifier and theinput/output control circuit 14 to output buffer 16 while the signalEXT.BI is inactive.

The internal clock signal CLK generated by the internal period settingcircuit 20 is applied to a control circuit 18 as a row selectingoperation activating signal (internal RAS). Control circuit 18 activatesthe row selecting operation activating signal in synchronization withthe clock signal CLK from internal period setting circuit 20 when thetest mode is designated by the external signal EXT.BI. Except thesepoints, the structure is the same as the conventional semiconductormemory device shown in FIG. 47, and corresponding portions are denotedby the same reference characters. Therefore, detailed descriptionthereof is not repeated.

FIG. 2 shows an example of the structure of internal period settingcircuit 20 shown in FIG. 1. Referring to FIG. 2, internal period settingcircuit 20 includes cascade connected plural stages (in FIG. 2, fourstages) of inverters 21 a to 21 d, and an NOR gate 21 e receiving anoutput signal from inverter 21 d and the test mode designating signalEXT.BI through an inverter 21 f. The number of stages of the inverters21 a to 21 d may be appropriately set in accordance with the period ofthe clock signal CLK to be generated.

Therefore, when the circuit shown in FIG. 2 is used as the internalperiod setting circuit 20, the period of the clock signal CLK, which isthe output therefrom is fixed at a prescribed period set in advance.

FIG. 3 shows another example of the structure of the internal periodsetting circuit 20 shown in FIG. 1. Referring to FIG. 3, the internalperiod setting circuit 20 b includes K-1 (K is an odd number) delay timevariable elements 110.1 to 110.K-1 connected in series with a biasgenerating circuit 100. The internal period setting circuit 20 b furtherincludes a delay time variable element 110.K which is connected to thelast stage of the aforementioned delay time variable elements connectedin series, start of operation of which is controlled by the test modedesignating signal EXT.BI.

Bias generating circuit 100 includes a P channel MOS transistors 101 and102, as well as N channel MOS transistors 103 and 104. P channel MOStransistor 101 and N channel MOS transistor 103 are connected in seriesbetween a power supply line 121 and a ground potential line 122. Pchannel MOS transistor 102 and N channel MOS transistor 104 areconnected in series between power supply potential line 121 and groundpotential line 122. P channel MOS transistors 101 and 102 have theirgates commonly connected and to the drain of P channel MOS transistor101. More specifically, P channel MOS transistors 101 and 102 constitutea current mirror circuit. N channel MOS transistor 103 receives at itsgate an internal clock period control signal FS. N channel MOStransistor 104 has its gate connected to its drain.

To N channel MOS transistor 103, a current Ia which increases/decreasesin accordance with the internal clock period control signal FS flows.MOS transistors 103 and 101 are connected in series, MOS transistors 101and 102 constitute a current mirror circuit, and MOS transistors 102 and104 are connected in series. Therefore, the same current Ia flowsthrough these four MOS transistors 101 to 104, provided that the MOStransistors 101 and 102 have the same transistor size.

Delay time variable element 110.1 includes P channel MOS transistors111.1 and 112.1 and N channel MOS transistors 113.1 and 114.1 connectedin series between power supply potential line 121 and ground potentialline 122. P channel MOS transistor 111.1 has its gate connected to thegate of P channel MOS transistor 102 of bias generating circuit 110. MOStransistors 112.1 and 113.1 have their gates commonly connected, and MOStransistors 112.1 and 113.1 constitute an inverter 115.1.

N channel MOS transistor 114.1 has its gate connected to the gate of Nchannel MOS transistor 104 of bias generating circuit 100. This is thesame in other delay time variable elements 110.2 to 110.K-1. Inverters115.1 to 115.K-1 are connected in series. An output of NAND circuit 115.K is connected to the input of inverter 115.1.

The operation of the internal period generating circuit 20 b shown inFIG. 3 will be described. P channel MOS transistors 111.1 to 111.K havetheir gates both connected to the gate of P channel MOS transistor 102,and N channel MOS transistors 114.1 to 114.K are connected to the gateof N channel MOS transistor 104. Therefore, the current Ia correspondingto the internal clock period control signal FS flows through each of thedelay time variable elements 110.1 to 110.K.

When the internal clock period control signal FS increases and thecurrent Ia increases, the time of inversion of each of the inverters115.1 to 115.K-1 as well as the time of inversion of NAND circuit 115.Kbecomes shorter, and thus period of oscillation of the internal periodsetting circuit 20 b becomes shorter.

When the internal clock period control signal FS decreases and thecurrent Ia decreases, the time for inversion of the inverters 115.1 to115.K-1 and of NAND circuit 115.K becomes longer, and thus the period ofoscillation of internal period setting circuit 20 b becomes longer.

While the test mode designating signal EXT.BI is at the “L” level, theNAND circuit 115.K is inactive, and hence the output from internalperiod setting circuit 20 b is stopped.

The above described structure enables the operation of the internalperiod setting circuit 20 b, of which start and stop is controlled bythe test mode designating signal EXT.BI, and period of oscillationcontrolled by the internal clock period control signal FS.

FIG. 4 is a schematic diagram showing an example of the structure ofbuffer input signal control circuit 24 shown in FIG. 1.

Buffer input signal control circuit 24 includes an NAND circuit 240receiving as inputs, an output signal Dout from sense amplifier andinput/output control circuit 14 and an inverted signal of test modedesignating circuit EXT.BI, and an NAND circuit 242 receiving as inputsthe internal clock signal CLK output from the internal period settingcircuit 20 and the test mode designating signal EXT.BI. While the testmode designating signal EXT.BI is at the “L” level, NAND gate 240 isopened and a signal Dout is output.

Meanwhile, if the test mode designating signal EXT.BI is at “H” level,NAND gate 242 is opened, and the clock signal CLK is output.

By the above described structure, it is possible to continuously operatethe output buffer circuit even in the test mode period, and henceacceleration test of the output buffer and the internal circuitry can besimultaneously performed in the acceleration test such as the burn intest mode.

In the present embodiment, only the output buffer circuit 16 is set tothe operative state during the test period. However, a structure may bepossible in which both the input buffer circuit 15 and the output buffercircuit 16 are set to the operative state in the test mode period.

FIG. 5 is a diagram of signal waveforms showing the operation of thepresent invention.

After the test mode designating signal EXT.BI rises from the “L” levelto the “H” level, the semiconductor memory device 1 operates inaccordance with the internal clock signal CLK which is the output frominternal period setting circuit 20, a word line is driven, and potentialdifference between a bit line pair (BL, /BL) is amplified correspondingto the memory cell information. Therefore, when a number ofsemiconductor memory devices are arranged on one board and test ofoperation is to be performed simultaneously as shown in FIG. 45, theinternal clock signal in each semiconductor memory device can maintainprescribed period and prescribed waveform, regardless of any distortionin the waveform of the external test signal.

Therefore, by applying test mode designating signal EXT.BI as anexternal trigger, each semiconductor memory device 1 can operate at highspeed without any influence by the parasitic capacitance or the likeexisting on the board.

Second Embodiment

FIG. 6 is a schematic block diagram showing a structure of semiconductormemory device 1 in accordance with the second embodiment of the presentinvention.

The present invention differs from the first invention in that theinternal clock period control signal FS for controlling the period ofinternal clock signal St CLK, which is the output from internal periodsetting circuit 20 is applied not externally but from a period settingcircuit 26 which can store, in non-volatile manner, the value of thesignal FS.

FIG. 7 is a schematic block diagram showing connection to the periodsetting circuit 26 and to the internal period setting circuit 25 b shownin FIG. 3.

To the gate of N channel MOS transistor 103 in bias generating circuit100, an output from period setting circuit 26 is input.

FIG. 8 shows details of the structure of period setting circuit 26.

Between a constant current source 242 and the ground potential,resistances 234, 236, 238 and 240 are connected in series. A fuseelement 228 is connected to resistance 234, fuse element 230 toresistance element 236 and fuse element 232 to resistance 238, parallelto each other. A potential at a node between constant current source 242and resistance 234 is output as the internal clock period control signalFS.

By blowing fuse elements 228, 230 and 231 by means of laser trimming orthe like, the composite value of resistance values of the resistancesviewed from the side of the constant current source 242 changes, andhence the value of the internal clock period control signal FS can bechanged.

Specifications such as conditions of operation of the semiconductormemory device differ dependent on types. If the design differs,conditions for testing must also be changed. However, according to thepresent embodiment, the period of the internal clock CLK during the testmode can be flexibly and easily changed corresponding to the types ofthe semiconductor memory devices.

In the above described embodiment, a method has been described whichproduces test time for acceleration test such as burn in test byincreasing the speed of the period of the internal clock signal CLK asthe test condition during test mode period. In order to changeacceleration condition in the acceleration test, in addition to themethod of increasing the period of the internal clock signal CLK, amethod may be used in which the internal power supply voltage Vci whichis lowered from the external power supply voltage Vce and supplied tothe internal circuitry, is increased to the level of the external powersupply voltage.

FIG. 9 is a block diagram showing a structure of an internal powersupply voltage supplying circuit allowing setting of the above describedacceleration condition.

Between the external power supply voltage Vce and the ground potential,P channel MOS transistor 246 and a load 250 are connected in series. Tothe gate of P channel MOS transistor 246, an output of a differentialamplifier 244, which receives as inputs an output Vref from a referencevoltage generating circuit (not shown) and a knife power supply voltageVci, is input. Internal power supply voltage Vci is taken out as thepotential at the node between P channel MOS transistor 246 and load 250.The circuit forms a negative feedback loop by the output value ofinternal power supply voltage Vci, and hence a function of holding thevoltage Vci at the reference voltage Vref. Between the output ofdifferential amplifier 244 and the ground potential, an N channel MOStransistor 248 is connected, and its gate potential is controlled by thetest mode designating signal EXT.BI. Namely, during the test mode, testmode designating signal EXT.BI attains to the “H” level and N channelMOS transistor 248 is rendered conductive, so that the gate potential ofP channel MOS transistor 246 is pulled down to the ground potential.Therefore, P channel MOS transistor 246 is rendered fully conductive,and internal power supply voltage Vci is pulled up to the external powersupply voltage Vce.

FIG. 10 shows the difference between the normally used region andacceleration test region, in which the abscissa represents the externalpower supply voltage and the ordinate represents the internal powersupply voltage.

In the normal use region, the internal power supply voltage holds aconstant value even when there is fluctuation of the external powersupply voltage. However, in the acceleration test region, the internalpower supply voltage coincides with the external power supply voltage.

Therefore, the internal circuitry of the semiconductor memory device 1operates with the external power supply voltage which is higher than inthe normal operation, and hence burn in test or the like can beperformed under accelerated condition.

Third Embodiment

FIG. 11 is a schematic block diagram showing a structure ofsemiconductor memory device 1 in accordance with the third embodiment ofthe present invention.

Different from the first embodiment, the test mode designating signalEXT.BI is not directly applied from the outside but a test mode controlcircuit 19 is provided which receives external control signals /RAS,/CAS and /WE as well as address signals A0 to Ai and detects designationof the test mode based on the combination of these signals.

In an on-wafer test or the like, it is possible to input a test signalor the like from an external terminal for testing. However, if thesemiconductor memory device 1 has already been accommodated in a moldpackage as a finished product, input of external control signal must beprovided through an external pin.

FIG. 12 shows an example of a specific structure of control circuit 18and test mode control circuit 19 shown in FIG. 11. Referring to FIG. 12,control circuit 18 includes an RAS buffer 30 receiving external controlsignal /RAS (EXT./RAS) applied to external control signal input terminal4 for outputting internal row address strobe signal /RAS; a CBRdetecting circuit 31 receiving external control signals EXT./RAS andEXT./CAS applied to external control signal input terminals 4 and 5,respectively, for detecting setting of a CBR condition (in whichexternal control signal EXT./CAS is pulled down to the low level beforethe fall of the external control signal EXT./RAS); a one shot pulsegenerating circuit 32 responsive to the CBR detection signal from CBRdetector 31 for generating a one shot pulse signal; a timer 33 which isactivated in response to the CBR detection signal from CBR detector 31for applying activating signal to the one shot pulse generating circuit32 at every prescribed time period while the CBR detecting signal isactive; a test mode setting circuit 80 receiving external controlsignals EXT./WE, EXT./OE, EXT./RAS and EXT./CAS applied to externalcontrol signal input terminals 2 to 5 and when these external controlsignals satisfy a WCBR condition (Write Cas Before Ras condition:EXT./WE is at a high level and CBR condition is satisfied) andprescribed address signal satisfies a super Vcc condition (a potentialhigher than the Vcc potential, which is the normal high level) issatisfied, indicating that the test mode has been set; and aself-refresh mode setting detecting circuit 34 receiving EXT./RAS,EXT./CAS and EXT./WE for outputting a self-refresh mode designatingsignal Φss indicating setting of the self-refresh mode when thesecontrol signals satisfy a prescribed condition.

One shot pulse generating circuit 32 generates a one shot pulse signalwhich is kept active for a prescribed time period, in response toactivation of the CBR detection signal from CBR detector 31 and toactivation of a signal (refresh designating signal) from timer 33.

Control circuit 18 further includes a 2-input NOR gate 35 receiving theinternal row address strobe signal /RAS output from RAS buffer 30 andCBR detecting signal output from CBR detector 31; a 2-input AND circuit50 receiving CBR detection signal output from CBR detector 31 andself-refresh designating signal oss from self-refresh mode settingcircuit; a 2-input OR gate 52 receiving an output from AND circuit 50and test mode designating signal BI from test mode setting circuit 80; atransfer gate 38 responsive to a clock signal CLK from internal periodsetting circuit 20 for selectively passing the output signal from ORgate 52; a 2-input AND gate 39 receiving the clock signal CLK frominternal period setting circuit 20 and the output signal from transfergate 38; a 2-input AND gate 44 receiving an inverted signal of theoutput signal from OR gate 52 and the output signal from one shot pulsegenerating circuit 32; a 3-input OR gate 40 receiving the output signalfrom NOR gate 35 and output signals from AND gates 39 and 44; a 2-inputOR gate 41 receiving the output signal from AND gate 44 and the outputsignal from AND gate 39; and an RAS control circuit 42 for activatingcircuits related to row selecting operations at a prescribed timing, inresponse to the output signal ΦRAS from OR gate 40. In FIG. 12, RAScontrol circuit 42 controls activation/inactivation of row decoder 12.

NOR gate 35 outputs a high level signal when the signal /RAS from RASbuffer 30 is at the low level and the output signal from CBR detector 31is at the low level. More specifically, in the normal operation (whenCBR condition is not set), the NOR gate inverts and outputs the signalfrom RAS buffer 30. When the CBR condition is set, NOR gate 35 is set tothe inactive state of low level, regardless of the logic level of theoutput signal from RAS buffer 30. Therefore, when the CBR condition isset, the row selecting operation by the control of external controlsignal EXT./RAS is inhibited.

AND gate 50 outputs an active signal of high level, when the CBRdetection signal from CBR detector 31 is at the active state of highlevel, and the self-refresh mode designating signal Φss fromself-refresh mode setting circuit 34 is at the active state of highlevel. OR gate 52 outputs an active signal of high level when theoutput. from AND gate 52 is at the high level or when the test modedesignating signal BI from test mode setting circuit 80 is the activestate of high level. Namely, OR gate 52 outputs an active signal of highlevel only when the self-refresh mode or the test operation mode isdesignated and the word lines are to be successively selected.

Transfer gate 38 is formed, for example, of a P channel MOS transistor,and it is rendered conductive when the clock signal CLK from internalperiod setting circuit 20 is at the low level. Therefore, even when theclock signal CLK is at the high level when termination of the test modeis designated, the clock signal CLK is prevented from immediatelyfalling to the low level. The test mode operation is completed after thefall of the clock signal CLK to the low level. Thus destruction ofmemory cell data caused by inappropriate word line selection can beprevented. Therefore, the transfer gate 38 has a function of a latchcircuit latching the output signal from AND gate 37 at every rise of theclock signal CLK.

OR gate 40 outputs a word line selecting operation activating signal(internal RAS signal) ΦRAS which attains to the active state of highlevel when the output signal from AND gate 39, the output signal fromAND gate 44 or the output signal from NOR gate 35 is set to the highlevel. The signal ΦRAS is applied to RAS control circuit 42. In FIG. 12,the RAS control circuit 42 is shown to control only the row decoder 12.However, it also controls operations of other sense amplifier circuits,bit line equalizing/precharging circuit (not shown) and so on.

The output signal from OR gate 41 is applied to the internal addressgenerating circuit 10. Internal address generating circuit 10 incrementsor decrements the address value indicated by the address signal outputtherefrom at every fall of the output signal from OR circuit 41.

Therefore, in the semiconductor memory device 1 controlled by thecontrol circuit 18 having the above described structure, whenself-refresh mode or test mode is designated by the external controlsignals and the address signals A0 to Ai, word lines are selectedsuccessively by the internal clock signal CLK which is the output signalfrom internal period setting circuit 20, and refreshing operation of thememory cells belonging to the row designated by the internal addressgenerating circuit 10 is performed.

In FIG. 12, the internal clock period control signal FS is adapted to beapplied from a specific control pin, in order to externally control theperiod of the internal clock signal CLK which is the output signal frominternal period setting circuit 20. However, the value of the internalclock period control signal may be set by the test mode setting circuit80 based on the combination of prescribed address signals A0 to Ai andit may be output to the internal period setting circuit 20.

Fourth Embodiment

FIG. 13 is a schematic block diagram showing a structure of thesemiconductor memory device 1 in accordance with the fourth embodimentof the present invention.

The present embodiment differs from the first embodiment in that aninternal period circuit 70 is provided which receives, when the testmode is designated by the test mode designating signal EXT.BI, aninternal row strobe address signal øRAS generated corresponding to anexternally applied external clock signal, for example, the external rowstrobe signal EXT./RAS and generates an internal clock signal CLKsynchronized therewith is provided.

As already described with respect to the prior art, when a plurality ofsemiconductor memory devices are to be tested in parallel, theexternally applied external clock signal has its waveform distortedbecause of signal propagation delay on the test board. In the presentembodiment, an internal clock signal synchronized with the externalclock signal is generated by the internal synchronizing circuit 70 inorder to shape the waveform of the internal clock signal controlling theoperation of the internal circuitry of the semiconductor memory device1.

Internal row strobe signal ΦRAS is assumed to be an internal signalderived from the external row strobe signal EXT./RAS passed through theRAS buffer circuit 30, as in the control circuit 18 shown in FIG. 12.

The internal synchronizing circuit 70 may have a structure of a phaselocked loop circuit (PLL circuit) or a delay locked loop circuit (DLLcircuit).

FIG. 14 is a schematic block diagram showing a structure in which a DLLcircuit is used as the internal synchronizing circuit 70.

Referring to FIG. 14, the DLL circuit includes clock buffers 91 and 96,a phase comparator 92, a charge pump circuit 93, a loop filter 94 and avoltage controlled delay circuit 95.

Clock buffer 91 includes, as shown in FIG. 15, M (M is a positiveinteger) inverters 91.1 to 91.M connected in series, amplifies theexternal clock signal øRAS and outputs a clock signal ECLK. Clock signalECLK is applied to phase comparator 92 and voltage controlled delaycircuit 95. The size of the symbol of the inverters 91.1 to 91.Mrepresents the load driving capability of each of the inverters 91.1 to91.M, and the load driving capability of inverters 91.1 to 91.Mgradually increases toward the output end. The load driving capabilityof inverters 91.2 to 91.M of the succeeding stages are set to be thirdtimes or fourth times that of the load driving capability of inverters91.1 to 91.M-1 of the preceding stages.

The number M of inverters 91.1 to 91.M is set in accordance with thecapacity of phase comparator 92 and of voltage controlled delay circuit95.

Clock buffer 96 includes N (N is a positive integer) inverters 96.1 to96.N connected in series, amplifies the output ECLK′ from voltagecontrolled delay circuit 95 and outputs internal clock signals CLK andRCLK, as shown in FIG. 16. Internal clock signal CLK is supplied tocontrol gate circuit 22 as in the first embodiment. Clock signal RCLK isapplied to phase comparator 92. Load driving capability of inverters96.1 to 96.N constituting clock buffer 96 is also increased graduallytoward the output end, as in the clock buffer 90. The number N of theinverters 96.1 to 96.N is set in accordance with the load capacitance.The inverter (in the figure, 96.4) outputting the clock signal RCLK isselected such that phase difference between the external clock signalERAS and internal clock signal CLK has a prescribed value.

Phase comparator 92 shown in FIG. 14 will be described. FIG. 17 is acircuit diagram showing the structure of phase comparator 92. Referringto the figure, phase comparator 92 includes inverters 300 to 304,2-input NAND gates 305 to 310, a 3-input NAND gate 311 and 312, and a4-input NAND gate 313.

Inverters 300 receives clock signal ECLK from clock buffer 91. Inverter301 includes clock signal RCLK from clock buffer 96. NAND gate 305receives an output from inverter 300 and an output from AND gate 311,and provides a signal ø305. NAND gate 306 receives outputs from NANDgates 305 and 307 and provides a signal ø306. NAND gate 307 receivesoutputs from NAND gates 306 and 313, and NAND gate 308 receives outputsfrom NAND gates 309 and 313. 5 NAND gate 309 receives outputs from NANDgates 308 and 310, and provides a signal ø309. NAND gate 310 receives anoutput from inverter 301 and an output from NAND gate 312, and providesa signal ø310.

NAND gate 313 receives signals ø305, ø306, ø309 and ø310 from NAND gates305, 306, 309 and 310, and outputs a reset signal RES. NAND gate 311receives signals ø305, ø306 and RES from NAND gates 305, 306 and 313,and provides an up signal /UP through inverters 302 and 303. NAND gate312 receives signals ø309, ø310 and RES from AND gates 309, 310 and 313,and provides a down signal DOWN through inverters 304.

FIG. 18 is a timing chart showing relationship between each of clocksignal ECLK, clock signal RCLK, the output from 2-input NAND gate 305(signal ø305), the output from 2-input NAND gate 310 (signal ø310) theoutput from 4-input NAND gate 313 (reset signal RES), up signal /UP anddown signal DOWN.

Prior to the description of FIGS. 17 and 18, assume that the clocksignals ECLK and RCLK are both at the “H” level. In that case, gates 305and 310 provide “H” level, without fail. If the output from gates 306and 309 are at the “H” level, the output from gate 313 attains to “L”level, the outputs from gates 307 and 308 attain to the “H” level, andas a result, the output from gates 306 and 309 attain to the “L” level.Therefore, as long as the clock signals ECLK and RCLK are both at “H”level, gates 311 and 312 always output “H” level. After such a state,when the clock signals ECLK and RCLK are changed to the “L” level, theoutputs from gates 305 and 310 attain to the “L” level, and gates 306and 309 outputs “H” level.

Thereafter, let us assume that the clock signal ECLK rises first andthen clock signal RCLK rises delayed by the phase T1, as shown in FIG.18. In response to the rise of clock signal ECLK, the output ø305 ofgate 305 attains to the “H” level. However, as the clock signal RCLKremains “L” level, the output ø310 of gate 310 maintains the “L” level,and the output RES of gate 313 is not changed but maintained at “H”level. Therefore, the output from gate 311 changes to the “L” level.Meanwhile, the output from gate 312 is kept at the “H” level.

When the clock signal RCLK rises thereafter, the output ø310 of gate 310changes to the “H” level, four inputs to the gate 313 all attain “H”level, and the output RES from gate 313 changes to the “L” level. As aresult, the output from gate 311 again changes from “L” level to the “H”level, and gate 311 provides a pulse signal reflecting the phasedifference between clock signal ECLK and clock signal RCLK.

Meanwhile, the output from gate 312 changes to the “L” level, inresponse to the change of the output from gate 310 to the “H” level.However, as the output of gate 313 changes to the “L” level immediatelythereafter, it quickly returns to the “H” level. Therefore, gate 312outputs a pulse signal of a constant width not related to the phasedifference between the clock signals ECLK and RCLK.

When the clock signal RCLK falls first and the clock signal ECLK risesthereafter, the relation between the up signal /UP and down signal DOWNis reversed. Except this point, the operation is the same, and thereforedescription thereof is not repeated.

More specifically, as shown in FIG. 19, when the phase of clock signalECLK is delayed from that of clock signal CLK, phase comparator 92provides an up signal /UP having a prescribed pulse width and a downsignal DOWN having a pulse width corresponding to the phase difference,when the phases of clock signals ECLK and RCLK match each other, itprovides signals /UP and DOWN having the same pulse width, and when thephase of clock signal ECLK is advanced than the clock signal RCLK, itprovides the down signal DOWN having a prescribed pulse width and the upsignal /UP having the pulse width corresponding to the phase difference.

FIG. 20 is a circuit diagram showing a structure of charge pump 93 andloop filter 94 shown in FIG. 14. Referring to FIG. 20, charge pump 93includes a constant current source 123, a P channel MOS transistor 124,an N channel MOS transistor 125 and a constant current source 126connected in series between power supply potential line 121 and groundpotential line 122.

P channel MOS transistor 124 receives at its gate the up signal /UP, andN channel MOS transistor 125 receives at its gate the down signal DOWN.The connection node N 124 between P channel MOS transistor 124 and Nchannel MOS transistor 125 serves as an output node of charge pump 93.Loop filter 94 includes a resistance 127 and capacitor 12 connected inseries between output node N124 and ground potential line 122.

The operation of charge pump 93 and loop filter 94 shown in FIG. 20 willbe described. When the up signal /UP and down signal DOWN both attain tothe “L” level, P channel MOS transistor 124 is rendered conductive, Nchannel MOS transistor 125 is rendered non-conductive, and charges aresupplied to capacitor 127 through power supply line 122→constant currentsource 123→P channel MOS transistor 124→node N124→resistance 127. Thus,the voltage at node N124, that is, the control voltage VCOin graduallyrises.

Conversely, when the up signal /UP and down signal DOWN both attain tothe “H” level, P channel MOS transistor 124 is rendered non-conductive,N channel MOS transistor 125 is rendered conductive, and charges incapacitor 128 flows out through capacitor 128→resistance 127→node N124→Nchannel MOS transistor 125→constant current source 126→ground potentialline 122. Therefore, control voltage VCOin lowers gradually.

When up signal /UP attains to the “L” level and down signal DOWN attainsto the “H” level, MOS transistors 120 and 125 are both renderedconductive, the amount of charges flowing to node 124 becomes equal tothe amount of charges flowing out from node N124, and hence controlvoltage VCOin is not changed.

When the up signal /UP attains to the “H” level and down signal DOWNattains to the “L” level, MOS transistors 124 and 125 are both renderednon-conductive, node N124 is set to the floating state and controlvoltage VCOin does not change.

More specifically, when the clock signal ECLK has its phase delayed fromclock signal RCLK, the control voltage VCOin, which is the output fromcharge pump 93 and loop filter 94 lowers gradually, when the clocksignals ECLK and RCLK have their phases matching, the control voltagedoes not change, and if the phase of the clock signal ECLK is advancedthan that of clock signal RCLK, it increases gradually.

FIG. 21 is a partially omitted circuit diagram showing a structure ofthe voltage controlled delay circuit 95 shown in FIG. 14.

The circuit structure is basically the same as that of internal periodsetting circuit 20 b described with reference to FIG. 3. Therefore,details of the structure and operation are not repeated, and only thedifference will be described in the following.

More specifically, corresponding to the internal clock period controlsignal FS applied externally to control oscillating frequency of theinternal period setting circuit 20 b in FIG. 3, the output voltage VCOinfrom charge pump circuit 93 and loop filter 94 is input to the gate of Nchannel MOS transistor 101.

Meanwhile, in the internal period setting circuit 20 b, the output fromNAND circuit 141.K is connected to the input of inverter 145.1 in orderto perform oscillating operation. In the voltage controlled delaycircuit 95, the clock signal ECLK is input to the input of inverter145.1, and the output from NAND circuit 141.K is taken out as the clocksignal ECLK′.

Therefore, the operation of voltage controlled delay circuit 95 is asfollows.

As the gates of P channel MOS transistors 141.1 to 141.K are bothconnected to the gate of P channel MOS transistor 102 and the gates of Nchannel MOS transistors 144.1 to 144.K are connected to the gate of Nchannel MOS transistor 104, the current Ia flowing through N channel MOStransistors 101 and 104 also flows through each of the delay timevariable elements 140.1 to 140.K, in accordance with the control voltageVCOin.

When the current Ia increases as the control voltage VCOin rises, thetime for inversion of each of the inverters 145.1 to 145.K-1 and of NANDcircuit 145.K becomes shorter and delay time of the voltage controlleddelay time 95 becomes shorter.

When the control voltage VCOin reduces and the current Ia reduces, thetime for inversion of each of the inverters 145.1 to 145.K-1 and of NANDcircuit 145.K becomes longer, and the delay time of voltage controlleddelay circuit 95 becomes longer.

Based on the operation of each structural block described above, theoperation of the DLL circuit shown in FIG. 14 will be described. Whenthe phase of the clock signal RCLK is delayed from the clock signalECLK, phase comparator 92 provides the up signal /UP having the pulsewidth corresponding to the phase difference between clock signals ECLKand RCLK, and the down signal DOWN having a prescribed pulse width.Accordingly, charge pump 93 supplies charges to loop filter 94, controlvoltage VCOin increases accordingly, and the delay time of voltagecontrolled delay circuit 95 becomes shorter. Therefore, the phase of theclock signal RCLK advances, and the phase difference between the clocksignals ECLK and RCLK becomes smaller.

Conversely, when the phase of clock signal RCLK is advanced from clocksignal ECLK, the phase comparator 92 provides the down signal DOWNhaving the pulse width corresponding to the phase difference betweenclock signals RCLK and ECLK, and the up signal /UP having a prescribedpulse width. Accordingly, charges flow out from the loop filter 94 tothe charge pump 93, so that control voltage VCOin lowers and the delaytime of voltage control delay circuit 95 becomes longer. Thus the phaseof the clock signal RCLK is delayed, and the phase difference betweenclock signals RCLK and ECLK becomes smaller. By the repetition of suchprocess, the phase difference between clock signals RCLK and ECLK iseliminated.

FIG. 23 is a timing chart showing the operation of semiconductor memorydevice 1 having an internal synchronizing circuit 70 performing theabove described operation.

When a plurality of semiconductor memory devices 1 are arranged on atest board, the externally applied clock signal EXT.CLK has its waveformdistorted as shown in FIG. 23, because of the signal propagation delay.

However, when the test mode designating signal EXT.BI attains to the “H”level and internal synchronizing circuit 70 starts its operation, theinternal clock signal CLK output from the circuit comes to have arectangular shape which is synchronized with the external clock signal.

Therefore, even when the external clock signal has distorted waveform onthe board, it does not affect the circuit operation in the semiconductormemory device.

Fifth Embodiment

FIG. 24 is a block diagram showing a structure of theinternal-synchronizing circuit 70 in semiconductor memory device 1 inaccordance with the fifth embodiment. After the test mode designatingsignal EXT.BI attains to the “H” level, the output of internal clocksignal CLK must be started in response to the first rise of the externalclock signal EXT.CLK. The reason for this is that it is necessary toprevent the destruction of memory cell data caused by inappropriate wordline selecting operation, and that when there is large phase differencefrom the external clock, the time necessary for the phase to be matchedwith the external clock differ in each semiconductor memory device 1, asthe internal timer starts oscillation arbitrarily.

In the third embodiment shown in FIG. 12, the test mode is designated bythe test mode designating signal by the latching operation of transfergate 38 and AND gate 39, the internal clock signal CAL is output afterthe rise of the first oscillating waveform. The above described problemcan be solved by the similar structure in the present embodiment also.

Alternatively, as shown in FIG. 24, it is possible to start theoperation of voltage controlled delay circuit 95 by a logic circuit 72detecting the rising edge of the first external clock signal, after thetest mode designating signal attains to the “H” level.

FIG. 25(a) is a schematic diagram showing an example of the structure oflogic circuit 72.

It is different from the voltage, controlled delay circuit shown in FIG.21 in that the variable delay element NAND circuit 145.K of the laststage is changed to an NOR circuit 145.K, which receives at one input anoutput from an S-R flip-flop circuit 160 which in turn receives the testmode designating signal EXT.BI and the external clock signal EXT.CLK,and receives at another input, the output from inverter 145.K-1.

When the test mode designating signal EXT.BI attains to the “H” level,and then the external clock signal EXT.CLK first attains to the “H”level, the output from S-R flip-flop circuit 160 changes from the “H”level to the “L” level. Therefore, the voltage controlled delay circuit95 starts its operation in response to the rise of the first externalclock signal EXT.CLK after the test mode is entered.

By the above described circuit structure, occurrence of inappropriateword line selecting operation can be prevented.

Sixth Embodiment

FIG. 26 is a schematic block diagram showing a structure of asemiconductor memory device 1 in accordance with the sixth embodiment ofthe present invention.

FIG. 27 is a block diagram of a main portion showing, in greater detail,the structure of the semiconductor memory device 1 shown in FIG. 26.

The present embodiment differs from the fourth embodiment in thefollowing points. First, test mode setting circuit 86 in a test modecontrol circuit 82 is adapted to receive external control signalsEXT./RAS, EXT./CAS and EXT./WE and address signals A0 to Ai and inresponse to detection of test mode designation, to output a test modedesignating signal BI, and internal synchronizing circuit 70 is adaptedto start its operation in response.

Secondly, a switch circuit 84 receives an output from internal periodsetting circuit 20 and an output from internal synchronizing circuit 70and outputs an output from internal synchronizing circuit 70 while thetest mode designating signal BI is active, and outputs as an internalclock, the output from internal period setting circuit 20 while the testmode designating signal BI is inactive and self-refresh mode designatingsignal Φss is active.

Except these points, the structure of the circuit in accordance with thepresent embodiment is the same as that shown in FIG. 12, and hencecorresponding portions are denoted by the same reference characters anddescription is not repeated.

In such a structure, when self-refresh mode is designated by thecombination of external control signals EXT./RAS, EXT./CAS andsemiconductor memory device 1 performs self-refresh operation inaccordance with the internal clock signal CLK which is the output fromexternal period setting circuit 20, and when the test mode is designatedby the external control signals EXT.RAS, EXT./CAS and EXT./WE andaddress signals A0 to Ai, the device performs test mode operation usingas internal clock signal CLK, the output from internal synchronizingcircuit 70 which is synchronized with the clock signal applied as theexternal row strobe signal EXT.RAS to the external terminal 4.

FIG. 28 is a timing chart showing the operation of the internalsynchronizing circuit 70 shown in FIG. 27.

When the external write enable signal EXT./WE is at the “H” level, theexternal control signals EXT./RAS and EXT./CAS satisfy the CBR conditionand external address signal EXT.Add satisfies the super Vcc condition,test mode setting circuit 86 detects designation of the test mode, andprovides the test mode designating signal BI of “H” level. As the testmode designating signal BI is input to the NOR circuit 52, a signal atthe “H” level is input to the transfer gate 38. Therefore, when theinternal-clock signal CLK which is the output of internal period settingcircuit 70 is input from switch circuit 84 to the AND circuit 39, theinternal row strobe signal ERAS in accordance with the internal clocksignal CLK is input to RAS control circuit 42, so that word lines areselected successively.

By the above described structure, in the test mode period, thesemiconductor device 1 operates in accordance with the shaped internalclock signal output from the internal synchronizing circuit, which issynchronized with the external clock signal.

Therefore, the distortion in the waveform of the external clock signalEXT.CLK does not directly affect the operation of the internal circuitryof the semiconductor memory device.

Seventh Embodiment

The sixth embodiment was adapted such that the internal clock signal CLKis supplied to the internal circuitry as a signal synchronized with theexternal clock signal EXT.CLK. However, in that embodiment, otherexternal control signals for performing test operation are supplied toeach semiconductor memory device 1 through data bus line SG on the testboard. FIG. 29 is a schematic block diagram showing a structure of thesemiconductor memory device 1 in accordance with the seventh embodiment.It is different from the sixth embodiment in that a self test circuit400 is contained in the semiconductor memory device 1.

Self test circuit 400 includes a test vector generating portion 402, aself test control portion 404 and a determining portion 406.

Test vector generating portion 402 includes a counter, an ROM or an LFSR(Linear Feedback Shift Register) for generating a pseudo random number.For example, an n bit LFSR is capable of generating 2^(n)−1 differentpseudo random test vectors. Self test control portion starts itsoperation by the test mode designating signal, and controls generationof test vectors in the test vector generating portion 402 and writingoperation to the memory cells. The test vector written in the memorycell is read to the determining portion 406 under the control of selftest control portion 404, in which it is compared with an expectedvalue, so that bit error is detected.

The aforementioned writing and reading may be performed alternately.However, it is possible to collect outputs of a plural times and compareonly once at the end of the test, in order to improve the efficiency intesting.

By the above described circuit structure, once the test mode isexternally designated, the semiconductor memory device 1 operates inaccordance with the internal clock signal CLK which is synchronized withthe external clock signal EXT.CLK, and continues self test operationuntil a bit error is detected, and hence efficiency in acceleration testsuch as the burn in test can be significantly improved.

Eighth Embodiment

FIG. 30 is a schematic block diagram showing a structure of thesemiconductor memory device 1 in accordance with the eighth embodimentof the present invention.

FIG. 31 is a block diagram of a main portion showing in greater detailthe structure of the semiconductor memory device 1 shown in FIG. 30.

The present embodiment differs from the sixth embodiment in that theinternal synchronizing circuit 70 in test mode control circuit 80 isreplaced by an internal multiplication circuit 72 which receives anexternal clock signal, for example, EXT./RAS for outputting an internalclock signal CLK by multiplying the period.

Except this point, the circuit structure is the same as that shown inFIG. 27, and corresponding portions are denoted by the same referencecharacters and detailed description is not repeated.

FIG. 32 is a schematic block diagram showing a structure of the internalmultiplication circuit 72 shown in FIG. 31. The structure of internalmultiplication circuit 72 differs from the internal synchronizingcircuit 70 shown in FIG. 14 in that it has a frequency divider circuit98 receiving the output signal RCLK from clock buffer 96 and divides thesame to have a prescribed ratio of division.

The output signal ECLK from clock buffer 91 and an output signal nRCLKfrom the frequency dividing circuit 98 are input to phase comparator 92.The ratio of division of frequency dividing circuit 98 is 16, a signalhaving sixteen times the period of the output signal ECLK′ from voltagecontrolled delay circuit 95 is input to the phase comparator 92, andcharge pump circuit 93 is controlled such that the received signal hasits phase matched with the signal ECLK which is in accordance with theexternal clock signal EXT./RAS.

Therefore, the internal clock signal CLK output from clock buffer 96comes to have the period one sixteenth that of the external clock signalEXT./RAS.

Namely, a signal obtained by multiplying the external clock signalEXT./RAS is output as the internal clock signal CLK.

Therefore, even when the external clock signal operates withsufficiently slow period, the internal clock signal CLK may operate athigh speed. The influence of the distortion in the waveform of externalclock signal EXT./RAS on the test board becomes more serious as theperiod of the external clock signal EXT./RAS is shorter, and henceinfluence of the distortion in the waveform on the test board can besuppressed by the above described structure.

In this embodiment, also, the period of the internal clock signal CLKcan be varied by changing the ratio of division of the frequencydividing circuit 98 by using the internal clock period control signalFS. This embodiment can also have the internal clock signal adapted tobe output to the external terminal such as the EXT./CAS terminal 5.

Further, by inputting the aforementioned multiplied internal clocksignal CLK to the output buffer circuit, a structure may be provided inwhich the output buffer circuit can be subjected to acceleration testsimultaneously.

FIG. 33 is a timing chart showing the operation of the eighthembodiment. In the same manner as in the sixth embodiment, the WCBRcondition is designated by the external control signals EXT./RAS,EXT./CAS and EXT./WE and by setting the address signal EXT./Add to thesuper Vcc condition, the device enters the test mode, and thereafter,the internal clock signal CLK which is obtained by multiplying theperiod of the EXT./RAS signal is output.

Ninth Embodiment

FIG. 34 is a schematic block diagram showing a structure of a paralleltester in accordance with the ninth embodiment of the present invention.

In the first to seventh embodiments, each semiconductor memory devicehas an oscillating circuit or a synchronizing circuit therein forshaping the distortion in the waveform of the external clock signal onthe test board.

As a method of modifying distortion in the waveform of the externalclock signal EXT.CLK on the test board, different from the abovedescribed method, a structure may be used in which a plurality ofsynchronizing circuits are provided on each board and the external clocksignal is shaped on the test board.

Further, in order to obtain similar effect, the test board may bedivided into a plurality of pieces, and test clock signal RCLK may begenerated on each piece of the test board in synchronization with theexternal clock signal EXT.CLK.

Referring to FIG. 34, a plurality of semiconductor memory devices 1 arearranged divided onto a plurality of test boards, and the test boardsTB₁ to TB_(n) have corresponding test board synchronizing circuits TSC₁to TSC_(n), respectively. The externally applied external clock signalEXT.CLK is synchronized and shaped by each test board synchronizingcircuit TSC_(i) and output as a test board testing signal to eachsemiconductor memory device.

When the external clock signal EXT.CLK has its waveform distorted by thesignal delay caused by the parasitic capacitance Cp on the test boardand the operation of the semiconductor memory device becomes differentfrom each other, conditions for acceleration such as in the burn in testcome to differ from device to device. However, the above describedstructure of the parallel tester can solve this problem.

Tenth Embodiment

From the first to ninth embodiments, structure of a semiconductor memorydevice and structure of a parallel tester which can operate even whenthe period of the external clock signal is made faster at the time ofacceleration test such as burn in test in accordance with the externallyapplied clock signal or test signal have been described.

In order to carry out parallel acceleration test or the like at highspeed, it is necessary to speed up the clock signal for operation and,specifically in the acceleration test of semiconductor memory devices,it is important to reduce time for writing data and for comparison ofdata with an expected value after reading.

The tenth embodiment shows a structure of a semiconductor memory devicewhich allows writing of memory information for testing at high speed tothe memory cells in the semiconductor memory device 1.

FIG. 35 is a cross section showing a structure of a memory cell portionin a typical DRAM. Referring to FIG. 35, a DRAM memory cell 614 includesa memory cell transistor including an N type high concentration layer606 to which a bit line 611 is connected and an N type highconcentration layer 606 to which a word line 605 and a storage node 609are connected, and a memory cell capacitor including a storage node 609storing charges, a dielectric film 615 and a cell plate 610 which is anopposing electrode of the capacitor.

Elements are separated by a separating oxide film 604 from each other,and P type well 603 and an N type well 602 are formed in substrate 1. Ptype well 603 is supplied with a potential through a P type highconcentration layer from an interconnection 613 so that its potential isfixed.

FIG. 36 is an equivalent circuit diagram of the memory cell portionshown in FIG. 35. Referring to FIG. 36, storage node 609 which is thecharge storing capacitor electrode of the memory cell is connected tothe P well 603 by a diode structure. It is possible to transfer chargesto storage node 609 through P well 603.

More specifically, referring to FIG. 35, by controlling potentials ofinterconnection 613 and cell plate 610 connected to P well through Ptype high concentration layer 607 independent from each other, itbecomes possible to introduce charges to storage node 609 from the sideof P well 603. The method of introducing charges will be described inthe following.

FIG. 37 is an illustration of a method of writing data at “H” levelcollectively to each of the memory cells.

In the following, a method of writing data at “H” level to the memorycells when the power supply voltage is 4 volt and cell plate voltage is2 volt will be described as an example.

Referring to FIG. 37(a), a positive voltage is applied frominterconnection 613 to P well 603. Thus positive charges can beintroduced to storage node 609. 3; The amount of charges introduced atthis time takes into account the forward voltage drop at the PN junctionbetween P well 603 and N type high concentration layer 606. At thistime, the potential at cell plate 610 is set to −1 V and positivepotential on the side of the P well is set to the following value:

(Positive potential on the side of the P well)=+1+(forward voltage dropof PN junction between P well 603 and N type high concentration layer606) (V).

By this setting, the potential at storage node 609 will be +1 V.

Referring to FIG. 37(b), a negative potential is supplied to P well 3through interconnection 613. At this time, the negative potentialssupplied must be low enough to prevent a leak current, when the N typehigh concentration layer 606 connected to the storage node 609 of theDRAM and the P well 603 is reversely biased, from hindering chargeholding characteristic of the memory cell, and it must be at a levelsufficient to keep low the leak current between N type highconcentration layers 606 of adjacent memory cells so that the leakcurrent does not hinder the charge holding characteristics of the memorycells. Further, the aforementioned negative potential must be at a levelwhich is sufficient to prevent the subthreshold current of the switchingtransistor of the memory cell from hindering charge holdingcharacteristic of the memory cell.

Referring to FIG. 37(b), the charges stored in storage node 609 aremaintained by the negative potential at cell plate 610.

Referring to FIG. 37(c), when the potential at cell plate 610 isincreased to +2 V, the storage node 609 facing thereto with a dielectricfilm 612 interposed has its potential increased by inductive coupling.Therefore, the memory cell is set to a state in which “H” data(corresponding to +4 V) is written.

FIG. 38 is an illustration of a method of writing “L” level datacollectively to the memory cells of the semiconductor memory device 1.

In the following, a method of writing “L” level data to the memory cellswhen the power supply voltage is 4 V and cell plate voltage is 2 V willbe described.

Referring to FIG. 38(a), a positive voltage is applied frominterconnection 613 to P well 603, and thus charges are introduced tostorage node 609. At this time, the amount of introduction takes intoconsideration the forward voltage drop at the PN junction between P well603 and N type high concentration layer 606. At this time, the potentialof cell plate 610 is set to +3 V and the positive potential on the sideof the P well is set to the following value:

(Positive potential on the side of P well) =+1+(forward voltage drop atPN junction between P well 603 and N type high concentration layer 606)(V).

By the above setting, the potential at storage node 609 will be +1 V.

Referring to FIG. 38(b), a negative potential is supplied throughinterconnection 613 to P well 603. At this time, the negative potentialsupplied must be low enough so that the leak current, when the N typehigh concentration layer 606 connected to storage node 609 in the DRAMand P well 603 is reversely biased, does not hinder the charge holdingcharacteristic of the memory cell, and it must be low enough so that theleak current between N type high concentration layers 606 of adjacentmemory cells does not hinder the charge holding characteristic of eachof the memory cells. Further, the negative potential must be maintainedlow enough so that the subthreshold current of the switching transistorof the memory cell does not hinder the charge holding characteristic ofthe memory cells.

In FIG. 38(b), the charges held in the storage node 609 is held by thenegative potential at cell plate 610.

Referring to FIG. 38(c), when the potential at cell plate 610 is loweredto +2 V, the storage node 609 facing thereto with dielectric film 612interposed experiences potential drop by coupling. Therefore, the memorycell is set to a state in which “L” level data is written.

FIG. 39 is a schematic block diagram showing an example of a circuitstructure of the semiconductor memory device 1 allowing the abovedescribed method of collective writing of data to the memory cells.

At crossings between bit line pairs 628 a to 628 h and word lines 624 ato 624 f, memory cells 622 are arranged. When information stored in amemory cell 622 arranged at a crossing between word line 624 a and bitline pair 628 a is to be read, the potential of word line 624 a is setto “H” level. Consequently, memory cell transistor is renderedconductive, and charges stored in memory cell capacitor causes potentialdifference at the potential of bit line pair 628 a. This small potentialdifference is amplified by sense amplifier 623, and it is coupled to I/Oline by selector 625, and hence data is externally read out.

In the semiconductor memory device 1 shown in FIG. 39, in normaloperation, the potential Vc of cell plate 610 is maintained at thepotential Vcp generated by a cell plate potential generating circuit520. The output Vcp from cell plate potential generating circuit isconnected to an interconnection 560 which is connected to the cellplates of memory cells by a switch circuit 530. Meanwhile, the potentialVw of P well 603 in each of the memory cells is held at a constant valueV_(BB) by a substrate potential generating circuit 522 in the normaloperation. The output of the substrate potential generating circuit isconnected to an interconnection 570 connected to the P well 3 of eachmemory cell by switch circuit 532.

When collective write operation of data to the memory cells describedwith reference to FIGS. 37 and 38 is to be performed, the outputs V_(CQ)and V_(WC) of cell plate potential/P well potential setting circuit 524are supplied to cell plate and P well 603 through switch circuits 530and 532, respectively. In response to the external control signal CCP,cell plate/P well potential setting circuit 524 controls the cell platepotential and the P well potential, and hence “H” level data or “L”level data are written collectively to the memory cell.

FIG. 40 is a schematic block diagram showing a circuit structureallowing test of such a DRAM that employs the above described datawriting method. In the cell plate potential/P well potential settingcircuit 524, data to be written to the memory cell of each of the memorycell array 620 divided into four is determined, writing operation isperformed, data is read from each memory cell array, and the read valueis compared with the expected value to detect a defect. At this time,the number of data read simultaneously may be arbitrarily set by circuitdesign or by employing an array multiple division architecture. Further,conversion of a plurality of data read simultaneously is performed bymatch detecting circuit 526 provided in semiconductor memory device 1.Thus the plurality of data read from respective memory cell arrays aredetermined to be matching or not matching.

Eleventh Embodiment

FIG. 41 is a schematic block diagram showing a structure of asemiconductor memory device in accordance with the eleventh embodimentof the present invention.

Different from the tenth embodiment, during the test mode period, thecell plate potential and the P well potential can be controlled byexternal terminals 580 and 582 through switch circuits 530 and 532.

When test operation is to be performed in the semiconductor memorydevice 1 having the above described structure, first, the test mode isdesignated by an external control signal, and by the test modedesignating signal BI, switching circuits 530 and 532 connect theexternal terminals 580 and 582 to the cell plate and to the P well,respectively.

By controlling the cell plate potential and the P well potential by anexternal tester, prescribed-memory cell data is written to each memorycell array 620. In this case, even when “H” data is written to all thememory cell arrays, the data read out may include both the “H” level and“L” level because of memory cell arrangement, as shown in FIG. 41. Forexample, when word line 624 a is activated, “H” level data is outputfrom memory cell 622. However, if the word line 624 b is activated, thebit line to which memory cell 622 is connected would be at “L” level.

Therefore, when it is to be compared with the expected value, such stateof inversion of the data must be recognized in advance on the side ofthe external tester. In FIG. 41, four bit line pairs, for example bitline pairs 628 a, 628 b, 628 c and 628 d as well as bit line pairs 628e, 628 f, 628 g and 628 h are connected through selector circuit 625through comparator 627. Therefore, when word line 624 a is activated,“H” data is output from memory cells 622 of bit line pairs 628 a and 628b. However, if the word line 624 b is activated, “L” data is output frommemory cells 622 of bit line pairs 628 a and 628 b.

Therefore, expected values of data to be read are stored in advance inthe external tester, and these values are input to the comparator 627 inadvance. Thereafter, reading operation is performed and transistor 626is activated so as to input the output from selector circuit 625 tocomparator 627, whereby comparison between each bit data and theexpected value can be carried out collectively, parallel to each other.

Each comparator outputs the result of comparison with the expected valueto the outside through a signal line DV. By using the test method in theabove described manner, it becomes possible to detect bit error of eachmemory cell collectively in parallel and hence the time necessary fortesting the semiconductor memory device 1 can be significantly reduced.

Twelfth Embodiment

In the tenth and eleventh embodiments, data for testing can be writtencollectively to the memory cells, and hence the time for the test of thesemiconductor memory device 1 can be reduced. However, the abovedescribed method of testing has a disadvantage that the influence ofinterference caused by the change in data pattern from the memory cells,that is, memory cell pattern dependency, cannot be detected. This isbecause only the same data can be written to the memory cells connectedto the same cell plate. Generally, the memory cells connected to anarbitrary bit line pair are connected to the same cell plate, and hencethis problem occurs.

FIG. 42 is a schematic block diagram showing the structure of thesemiconductor memory device 1 for solving the above described problem.Different from the first embodiment, the interconnection commonlyconnecting the cell plates from each other is divided into two sets, aswill be described in the following.

More specifically, a first cell plate interconnection 590 is connectedto the cell plates of memory cells connected to every other lines ofcells in a diagonal direction of the bit line pair and the word linesconnected to the memory cells.

By contrast, a second cell plate interconnection 512 is commonlyconnected to remaining memory cells which are not connected to the firstcell plate interconnection 731. Cell plate potential/P well potentialsetting circuit 524 is adapted to control the potential (V_(CQ1) andV_(WC)) of the first cell plate interconnection 590 and P wellinterconnection 570 as well as potentials (V_(CQ2) and V_(WC)) of thesecond cell plate interconnection 592 and P well interconnection 570independent from each other.

FIG. 43 shows a state in which test data is written to the memory cellscollectively in such a semiconductor memory device 1 having the abovedescribed structure.

Referring to FIG. 43, the first cell plate interconnection 731 is usedfor writing “L” level, and the second cell plate interconnection 732 isused for writing “H” level in accordance with the method described withreference to FIGS. 37 and 38, and the figure shows the data written torespective memory cells by this method. Referring to FIG. 43, since thecell plate is separated diagonally, “H” level memory cells and “L” levelmemory cells continue in diagonal stripes. Therefore, the states of thememory cells connected to one word line are alternating CHIN level and“L” level, and hence data of adjacent memory cells are inverted fromeach other. Therefore, influence of interference between memory cells onthe data can be detected.

Thirteenth Embodiment

FIG. 44 is a schematic block diagram showing a structure of thesemiconductor memory device 1 in accordance with the thirteenthembodiment.

In the twelfth embodiment, the potential between first cell plateinterconnection 520 and P well interconnection 570 and the potentialbetween the second cell plate interconnection 592 and P wellinterconnection 570 are controlled by cell plate potential/P wellpotential setting circuit 524. However, in the thirteenth embodiment,these are controlled by external terminals 580, 581, 582, as in theeleventh embodiment.

The present invention is similar to the eleventh embodiment except thatdifferent test data can be collectively written to the memory cellsconnected to the first cell plate 590 and the memory cells connected tothe second cell plate 522 independent from each other. Therefore,detailed description thereof is not repeated.

By the structure shown in FIG. 44, it becomes possible to write data tothe memory cells such that “H” level data and “L” level data arearranged alternately for the memory cells connected to the same wordline by means of an external tester, and hence influence of interferencebetween memory cells on the data can be detected.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A parallel tester, for performing operation tetof a plurality of semiconductor memory devices parallel to and insynchronization with each other in accordance with an externally inputexternal clock signal, wherein said plurality of semiconductor memorydevices are divided into a plurality of subgroups; said parallel testercomprising internal test clock generating means provided for each one ofsaid subgroups, for receiving said external clock signal and forgenerating a synchronized internal test clock signal; and a data busline for transmitting said internal test clock signal to each of thesemiconductor memory devices of said subgroup.